Ring hydrogenation of aromatic amines using Group 6 and Group 8 metals carried on a support is well known. Two aspects in the hydrogenation process are problematic. First, contaminants in the aromatic amine substrate can poison the catalyst thus impacting catalyst activity and catalyst life. Second, catalyst attrition can occur thereby resulting in catalyst loss and plugging of catalyst filtration equipment.
Representative patents which illustrate various processes for the hydrogenation of aromatic amines are as follows:
U.S. Pat. Nos. 2,606,925 and 2,606,927 disclose the hydrogenation of nitroaromatics and aromatic amines. The '925 patent shows the use of ruthenium oxide as a catalyst whereas the '927 discloses the use of cobalt on alumina.
U.S. Pat. Nos. 3,636,108 and 3,697,449 disclose the hydrogenation of aromatic compounds and, particularly, 4,4-methylenedianiline to produce a product referred to as PACM, using an alkali metal-moderated ruthenium catalyst. Alkali moderation is accomplished by depositing a ruthenium compound on a support from an aqueous solution of sodium or potassium bicarbonate, hydroxide, or the like. A wide variety of carriers such as calcium carbonate, rare earth oxides, alumina, barium sulfate, kieselguhr and the like are shown as candidate supports. The '449 patent discloses the in situ alkali moderation of the catalyst.
U.S. Pat. No. 4,754,070 discloses an improved process for the hydrogenation of methylenedianiline contaminated with catalyst poisoning impurities. A catalyst comprised of rhodium and ruthenium was found to be effective in the hydrogenation of a crude methylenedianiline, i.e., one containing oligomers. Alkali moderation via addition of lithium hydroxide activation was shown to be effective for the combined catalyst. Carriers suited for the rhodium/ruthenium catalyst included alumina, carbonates, etc.
U.S. Pat. No. 5,545,756 discloses a process for the hydrogenation of aromatic amines, whether mononuclear or polynuclear, using a catalyst of rhodium carried on a titania support. Examples of titania supports include TiA1.sub.2 O.sub.5, TiSiO.sub.4 and TiSrO.sub.3. The titania support permitted the use of rhodium alone as the active metal in the hydrogenation of crude methylenedianiline. Rhodium carried on titania in combination with ruthenium on alumina was also suited as a catalyst. Lithium hydroxide activation results in enhanced activity.